I think I'm missing something here. if you have a grammar rule R with children A and B, and a function in your recursive descent parser that corresponds to R, w

I think I'm missing something here. if you have a grammar rule R with children A and B, and a function in your recursive descent parser that corresponds to R, why can R not call the parser functions for A and B, which return AST nodes themselves, and then construct another AST node using the result of those? Where was the visitor pattern required here?